Corals stir their own water for oxygen, and the stirring fails before the heat does

Cilia beat faster as water warms, but a Science study found oxygen demand rising faster still. At 39C the cilia stopped and the coral died.

Corals move water across their own surface using cilia, generating tiny vortices that pull in oxygen. An international collaboration including Cesar Pacherres and Michael Kuhl of the University of Copenhagen exposed the stony coral Porites lutea to rising temperatures and watched what that machinery does under heat. At 27C the cilia moved normally. At 37C they began to slow. At 39C, about 102F, they shut down entirely and the coral died.

The mechanism is a trap rather than a simple threshold. Warming raises the coral's oxygen demand and the cilia beat faster in response, but as Kuhl puts it, the oxygen demand rose faster than the increased swirling of the water. Warmer water also holds less dissolved oxygen to begin with. So the animal accelerates its only pump into a supply that is shrinking underneath it.

The implication researchers draw is that deoxygenation, not bleaching alone, may be the thing doing the killing, which would explain why corals on the same reef die at different rates and why some die without ever expelling their algae. One quoted assessment is that oxygen could have been the smoking gun all along. Two honest caveats: this is a laboratory temperature ramp on one species, and the paper appeared in Science in May 2026, so what happened this week was the explainer, not the finding.

Source: Quanta